A circular economy for photovoltaic-grade silicon can achieve a critical CO₂ emissions reduction in Europe

Reducing the carbon intensity of photovoltaics

Photovoltaics is a decisive renewable energy source to achieve the decarbonization targets of the European Union in 2030 and 2050. Indeed, our electricity grids still heavily rely on fossil fuels in most countries and the average greenhouse gas emissions intensity related to electricity generation was just under 300 g_CO2/kWh in 2016 . In comparison, electricity generated by photovoltaic installations in Europe emits around 70 g_CO2/kWh when taking into account the whole lifecycle . It allows a drastic reduction but the PV industry could still reduce this value by increasing the efficiency of solar panels, or by reducing the emissions from the manufacturing and installation of the power plants.

For a photovoltaic project, whether large scale ground-mounted or small-scale rooftop, most of the energy consumption and the greenhouse gas emissions are related to the manufacturing of the photovoltaic modules . More precisely, within the PV panel manufacturing process, the sourcing of photovoltaic-grade silicon and its transformation into silicon wafers is what emits most of the greenhouse gases. If we were to reduce the reverse effects of PV on climate change, we need to focus on reducing the energy consumption and direct emissions of these upstream stages of the photovoltaic value chain. This is one of the objectives of ROSI Solar.

Current impact of silicon production

Producing photovoltaic-grade silicon (nicknamed PV-Si) is a 2-step process. First, the production of metallurgic-grade silicon consists in the transformation of quartz, wood and coal into silicon with the help of an arc furnace. This process is energy intensive and has direct emissions related to the chemical reaction. The output is a silicon with a purity of 98-99% (1N – 2N, so-called metallurgical grade Silicon). Secondly, this metallurgical-grade silicon needs to be purified in order to be used in the photovoltaic industry. This complex chemical reaction process allows to reach a purity up to 99,999999999% (11N). But it is a massive energy pit requiring more than 60 kWh of electricity for every kg of silicon. Given the fact that the main manufacturers are based in China where the electricity mix is carbon intensive, the production of 1 kg of photovoltaic-grade silicon from raw materials requires 100 kWh of electricity and emits 100 kg_CO2 (direct + indirect emissions).

At a global scale, we produced 500 000 tons of PV-Si in 2019, so the photovoltaic industry emitted 50 million tons of CO₂ just for this silicon material supply. It represents 115 GW  of annual installed capacity worldwide. In Europe, where 16 GW of PV capacity were installed in 2019, this means 7 million tons of CO2 where emitted due to this silicon material needs. It accounts for around 0.17% of the total emissions of the continent imported with PV modules mainly manufactured in China [5]. But let’s take a look in the future: the European Commission published in its energy strategy “A Clean Planet for all” [6] 9 scenarios for our energy mix. From 2030 to 2050, Europe would need to add 20 to 40 GW of photovoltaic capacity per year depending on the scenario. It translates into the emission of 8 to 17 million tons_CO2/year if there is no technological change. This accounts for more than 0.5% of the GHG emissions targets in 2030 and up to 3% in 2050! Reducing our reliance on this traditional source of photovoltaic-grade silicon is a major challenge for the energy transition to be compatible with our climate change goals.

A circular economy for silicon

A first solution to reduce the carbon emissions from the photovoltaic-grade silicon production is to recycle the silicon lost during the wafer sawing process. Indeed, when cutting silicon ingots into very thin wafers, the industry is currently losing 40% of the material as silicon dust mixed with the sawing liquid. This sludge is called kerf and even though it is composed of highly pure silicon particles, it is currently not recycled in the industry. Implemented at large scale, the recycling process for the kerf sludge developed by ROSI Solar would have a major impact on the emissions related to the production of the PV panels, especially in China.

A second way to tackle this challenge is to look at the overall lifecycle picture. A photovoltaic module has a lifespan of 20 to 35 years. After that, PV installations are dismantled and the panels need to be recycled. The challenge is then to properly separate the ultra-pure silicon contained in the PV cells from the other materials. The silicon content in a PV panel is around 3 %  (roughly, 1 panel of 18 kg contains 0.5 kg of silicon). Following the scenarios from the International Energy Agency’s Photovoltaic Power Systems Programme (IEA-PVPS), the annual PV waste discharged in Europe should reach about 150 000 tons per year in 2030 and 1 million tons per year in 2050. This means Europe can recover large amounts of ultra-pure silicon from this waste stream: 5 000 tons per year in 2030 and 30 000 tons per year in 2050. In other words, in 2050, recycling this silicon could cover 25% to 50% of the PV-Si needs in Europe with current technologies, and much more if kerf recycling is in place.

In both scenarios, ROSI Solar can recycle silicon with a very high purity: 99.999% (5N). This means this recycled silicon will require much less energy to reach the purity of photovoltaic-grade silicon: no need to produce metallurgical-grade silicon from raw materials and a more efficient purification stage. ROSI Solar demonstrated that PV-Si produced from recycled silicon can reduce the carbon emissions by 50% with the current production infrastructure.

Now, let’s go further by looking at the big picture: recycling the silicon from end-of-life photovoltaic modules in Europe at a large scale would require a lighter purification process, so a smaller and cheaper PV-Si factory. Moreover, the use of a lower carbon-intensive electricity compared to the Chinese grid would reduce even more the carbon emissions related to PV-Si production. Combined with the recycling of kerf sludge, a silicon recycling infrastructure based in Europe could reduce the carbon emissions of PV-Si production by up to 80%.

In the end, it is critical to establish an industrial infrastructure for recycling silicon in Europe. It is the most straightforward way to reduce the carbon emissions related to the PV industry. Moreover, it will allow to source the material directly in Europe. If we are to restart a PV manufacturing base in Europe with GW factories, this local source of photovoltaic-grade silicon will strengthen our ability to produce very low-impact PV modules. The opportunity is now and ROSI Solar is already planning the first industrial-scale recycling plants that will kick-start this transition to a circular economy for silicon.